An electrical balance duplexer (EBD) is a tunable RF frontend concept that addresses some of the key challenges of 4G and 5G mobile systems. The basic principle of the EBD is shown in FIG. 1. Duplexer isolation is achieved when the signals in paths 1 and 2 cancel and prevent the signal TX from appearing at the RX port. This cancellation is achieved by balancing the antenna port's impedance ZANT with a tunable on-chip impedance ZBAL, hence the name “electrical balance”.
Wireless devices now support an increasing number of 4G FDD bands, and a tunable integrated solution like this could potentially replace several fixed-frequency filters and switches. The EBD is also attractive for 5G in-band full-duplex (FD) communication systems. Whereas a filtering-based duplexer cannot isolate the TX and RX ports for the special case where fTX=fRX, the EBD's signal cancellation approach still works.
However, the entire EBD concept depends on the capability to tune ZBAL into balance with ZANT. This needs to happen not just at a single frequency, but across the entire bandwidth of both the TX and RX channels. Additionally, the impedance characteristic of the antenna will vary significantly over time due to interaction with the user and environment. Therefore, it is necessary to provide the EBD with a system for matching of the tunable impedance ZBAL to the antenna port's impedance ZANT.
A solution for tuning the tunable impedance is shown in FIG. 2. Here, a hybrid transformer of the EBD is represented as a 4-port circuit element. In clockwise order starting from the left side, the ports of the EBD are connected to the antenna, the low-noise amplifier (LNA, not shown) of the RX-chain, ZBAL and the power amplifier (PA, not shown) of the TX-chain. A tuner is provided on the antenna side for reducing antenna impedance variations present under environmental disturbances near the antenna near-field. With reduced variations, it is more feasible to tune the balance network to track the remaining variation and optimize the balance condition while simultaneously optimizing the PA efficiency.
Hybrid couplers connected to VX+ and VX−, are subsequently connected to amplitude and phase difference detectors that can measure the waves moving back and forth between the hybrid transformer and the load port (e.g. RX, BAL, PA, and ANT). In this way, it is possible to measure all transfer functions from any port to any other port. Such transfer functions can be captured in so-called S-parameters. The following generic 4-port matrix would define the transfer characteristics according to the waves that such a hybrid coupler-based sensing system can observe:
            [                                                  b              1                                                                          b              2                                                                          b              3                                                                          b              4                                          ]        =                  [                                                            S                11                                                                    S                12                                                                    S                13                                                                    S                14                                                                                        S                21                                                                    S                22                                                                    S                23                                                                    S                24                                                                                        S                31                                                                    S                23                                                                    S                33                                                                    S                34                                                                                        S                41                                                                    S                24                                                                    S                34                                                                    S                44                                                    ]            ⁡              [                                                            a                1                                                                                        a                2                                                                                        a                3                                                                                        a                4                                                    ]              ,where ai and bi, are the waves as defined by S-parameter theory for all ports, and port 1 is the ANT port, port 2 the PA/TX port, port 3 is the ZBAL port, and port 4 is the LNA/RX port.
A strategy is to observe all scattering parameters (S-parameters), and calculate from all those parameters the reflection required at the BAL port that will provide electrical balance, from which the BAL impedance needed for electrical balance can also be derived. For any given time instant, under the assumption that the antenna remains constant during a sufficiently long period in time for the algorithm to run, the waves are used to derive S-parameters, giving direct system-state information that can concurrently be used to calculate the optimized ZBAL that enables signal cancellation of a signal generated in the PA/TX.
This solution has the disadvantage that it is complex in design, and thus expensive to implement. Furthermore, the couplers have the disadvantage of being large components which are lossy when integrated on typical silicon technologies. This solution also does not allow for real-time balancing of the electrical balance duplexer, as it requires reducing antenna impedance variations before calculation of all the S-parameters requires tuning of both the antenna's port impedance and the tunable impedance.
Another solution for balancing an electrical balance duplexer is shown in WO 2013/063506 A2. This solution is also complex in design and requires the tuning of an antenna impedance and a tunable impedance.